1. Field of the Invention
The present invention relates to a photocatalyst material, a photocatalyst composition and a photocatalyst product.
2. Description of the Related Art
As is well known, when a photocatalyst material is irradiated with light having an energy larger than the bandgap thereof, electrons appear in the conductive band and holes appear in the valence band by photoexcitation. As a consequence, these electrons and holes are diffused to the surface of the powder and brought into contact with oxygen and moisture, with the result that these electrons are adsorbed and reduced to produce super oxide anions. The holes, on the other hand, oxidize moisture to form hydroxy radicals. These products resultantly exhibit sterilizing force, organic decomposability and hydrophilic ability through their redox reactions.
Here, examples of the “light having an energy larger than the bandgap” include ultraviolet rays and visible light. As the “light source”, sunlight, various lamps and light-emitting diodes are used.
As the photocatalyst materials, a titanium oxide (TiO2) powder has been primarily used so far. However, when it is intended to obtain the bandgap energy (wavelength of 380 nm or less) of a titanium oxide powder by sunlight, only about 2% of the light can be utilized. In view of this, much attention has been recently focused on tungsten oxide (WO3), instead of a titanium oxide powder, as photocatalyst materials capable of utilizing visible light region (400 to 800 nm) which is a major wavelength of sunlight.
For example, Patent Document 1 describes that a tungsten oxide film formed by a sputtering method is used as a photocatalyst. Here, even if the photocatalyst is a film, the effect of the photocatalyst is obtained. However, the photocatalyst having a film form is unsatisfactory to gain sufficient surface area and is therefore decreased in catalyst effect per unit volume. Also, because the sputtering method is a film-forming technology using vacuum, the equipment is large-scaled, leading to high cost. Moreover, the sputtering method has the problem that a film can be formed only on a highly heat-resistant material (base material) because the base material (material to be coated) is exposed to a high-temperature circumstance.
When the photocatalyst is made of a tungsten oxide powder, on the other hand, it has the following merits. Specifically, because the entire surface of the powder can be used as the catalyst surface, the catalyst effect per unit volume can be improved. Also, a method in which the powder is mixed with a resin and applied may be adopted and it is therefore unnecessary to expose the base material to a high-temperature circumstance and the powder can be applied to any place. In order to increase the catalyst surface per unit volume, the powder is preferably made into microparticles having an average particle diameter of 1 μm or less.
As the method used to obtain microparticles of a tungsten oxide powder, for example, Patent Document 2 is known. Examples of the method of heat-treating ammonium paratungstate (hereinafter, referred to as APT) in the air are given in this Patent Document, Paragraphs 0008 and 0009. This method succeeds in obtaining microparticles having a BET specific surface area of 3 to 5 m2/g and an average particle diameter of 0.2 to 0.3 μm when the specific gravity of tungsten oxide is 7.3.
In the meantime, examples of the light source used to excite a photocatalyst include, as mentioned above, sunlight, various light-emitting diodes and various lamps. Here, the photocatalyst is excited by a specified wavelength to develop its catalytic effect. Therefore, if the wavelength of the light source does not accord to the exciting wavelength of the photocatalyst, sufficient characteristics cannot be obtained. For example, Patent Document 3 for improving such a disadvantage is known. In Patent Document 3, a method is disclosed in which a light-emitting material is made to emit light having a wavelength different from that of a light source by mixing the photocatalyst with the light-emitting material (fluorescent body), to excite the photocatalyst by the light having that wavelength.
According to Patent Document 3, there is disclosed such a fact that even if a TiO2 powder which is scarcely excited by the visible light is used, this powder exhibits the ability of decomposing formaldehyde under daylight color (under sunlight). However, the decomposing ability thereof is very poor. To mention in more detail, the method of Patent Document 3 takes 24 hours to decompose 50 ppm of formaldehyde.
On the other hand, much attention has been recently focused on tungsten oxide (WO3) as photocatalyst materials which are used in the visible light region (400 to 800 nm) as mentioned above. According to this photocatalyst, a certain level of catalyst characteristics is, indeed, obtained in the visible light region.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2001-152130
Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2002-293544
Patent Document 3: Jpn. Pat. Appln. KOKAI Publication No. 2002-346394
Patent Document 4: Jpn. Pat. Appln. KOKOKU Publication No. 4-42057